1. Field of the Invention
The present invention relates to a method for removing a metal impurity, such as aluminum, from a quartz component part used in a heat processing apparatus of the batch type for performing a heat process on target substrates, such as semiconductor wafers, and particularly to a technique utilized in the semiconductor process field. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target substrate, such as a semiconductor wafer or a glass substrate used for an FPD (Flat Panel Display), e.g., an LCD (Liquid Crystal Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target substrate.
2. Description of the Related Art
As a heat processing apparatus used for performing a process for manufacturing semiconductor devices, there is a vertical heat processing apparatus, which is a heat processing apparatus of the batch type. A vertical heat processing apparatus of this kind includes a heating furnace formed of a vertical reaction container opened at the bottom and a heater surrounding the reaction container. A number of semiconductor wafers (which will be referred to as wafers) are supported at intervals in the vertical direction on a wafer holder called a wafer boat. The wafers supported on the wafer boat are loaded into the reaction container from below, and are subjected to a heat process, such as oxidation, diffusion, or film formation using CVD (Chemical Vapor Deposition).
In a vertical heat processing apparatus of this kind, some of the component parts, such as a reaction container, a wafer boat, and a heat-insulating unit (heat-keeping unit), are typically made of quartz. However, in general, quartz products originally contain metal impurities, such as aluminum, which can be discharged when the products are heated. Accordingly, in the case of a vertical heat processing apparatus, when the quartz component parts are heated inside the reaction container, metal impurities are scattered into the heated atmosphere inside the reaction container and may be partly deposited on wafers being processed.
In recent years, as the film thickness and the size of semiconductor devices are being smaller, a conventionally negligible amount of metal impurities, such as aluminum, deposited on wafers can deteriorate characteristics of semiconductor devices. Accordingly, it is required to provide techniques for preventing contamination of wafers due to metal impurities, such as aluminum.
Patent Document 1 (Jpn. Pat. Appln. KOKAI Publication No. 2002-313787: paragraphs 0017 to 0018) discloses a technique for removing copper from the surface of a quartz component part, such as a reaction container. According to this technique, before a heat processing apparatus is used for performing a heat process on product wafers, hydrogen chloride gas and oxygen gas are supplied into the reaction container set to have a heated atmosphere. However, as described later with reference to experimental results, the present inventors have found that aluminum removal is insufficient where the technique disclosed in Patent Document 1 is used.